Iron silicide root formation in carbon nanotubes grown by microwave PECVD

Aligned carbon nanotubes have been grown using microwave plasma enhanced chemical vapor deposition (PECVD). The carbon nanotubes are nucleated from iron catalyst particles which, during growth, remain adherent to the silicon substrates. By analysis with high-resolution electron microscopy, we observe iron silicide roots penetrating into the silicon substrate at the interface of the catalyst particles and the substrate, thus providing strong adhesion of the carbon nanotubes onto the substrate. The iron silicide roots assist in the attachment of the catalyst particles to the substrate and play a role in the evolution of the catalyst particle morphology and resulting base growth mode. Carbon nanotubes grown by microwave PECVD could exhibit superior electrical and thermal transport properties over other PECVD processes, so an understanding of the growth mechanism is important for utilization in device applications.     

Selected-area growth of carbon nanotubes by the combination of focused ion beam and chemical vapor deposition techniques.

In this article, we report a technique for growing carbon nanotubes in a more controllable fashion, which enables us to synthesize nanotubes directly in various forms of designed patterns. This nanofabrication process is based on a combination of focused ion beam (FIB) and chemical vapor deposition (CVD) techniques. In this process, arrays of conductive patterns were first deposited on silicon substrates by directing a gaseous compound (C(9)H(16)Pt) via the capillary needle-sized nozzles within a FIB system. The substrates were then coated with catalyst and further modified by the FIB to localize the position of the catalyst. Finally, the growth of carbon nanotubes on the designed substrates was carried out by CVD of hydrocarbon gases. This fabrication technique has the advantage of positioning carbon nanotubes in selected locations. This may open up opportunities for the direct synthesis of carbon nanotubes onto almost any substrate material, thus allowing fabrication of carbon nanotube-based devices.     

Simple Dip-Coating Process for the Synthesis of Small Diameter Single-Walled Carbon Nanotubes-Effect of Catalyst Composition and Catalyst Particle Size on Chirality and Diameter

We report on a dip-coating method to prepare catalyst particles (mixture of iron and cobalt) with a controlled diameter distribution on silicon wafer substrates by changing the solution's concentration and withdrawal velocity. The size and distribution of the prepared catalyst particles were analyzed by atomic force microscopy. Carbon nanotubes were grown by chemical vapor deposition on the substrates with the prepared catalyst particles. By decreasing the catalyst particle size to below 10 nm, the growth of carbon nanotubes can be tuned from few-walled carbon nanotubes, with homogeneous diameter, to highly pure single-walled carbon nanotubes. Analysis of the Raman radial breathing modes, using three different Raman excitation wavelengths (488, 633, and 785 nm), showed a relatively broad diameter distribution (0.8-1.4 nm) of single-walled carbon nanotubes with different chiralities. However, by changing the composition of the catalyst particles while maintaining the growth parameters, the chiralities of single-walled carbon nanotubes were reduced to mainly four different types, (12, 1), (12, 0), (8, 5), and (7, 5), accounting for about 70% of all nanotubes.     

Synthesis and self‐assembly of polystyrene‐grafted multiwalled carbon nanotubes with a hairy‐rod nanostructure

Polystyrene‐grafted multiwalled carbon nanotubes (PS‐g‐MWNTs) with a hairy‐rod nanostructure were synthesized by the in situ free‐radical polymerization of styrene in the presence of multiwalled carbon nanotubes (MWNTs) terminated with vinyl groups. To quantitatively study the molecular weight and composition of polystyrene (PS) chains in PS‐g‐MWNTs, PS‐g‐MWNTs were fully defunctionalized by hydrolysis. The results showed that 1 of every 100 carbon atoms in MWNTs was functionalized at the tips and outer walls of the carbon nanotubes and grafted by PS with a weight‐average molecular weight of 9800 g/mol; therefore, a uniform thin layer (ca. 8–10 nm) of a PS shell was formed on the outer wall of MWNTs. PS‐g‐MWNTs were soluble in dimethylformamide and tetrahydrofuran. The thermal stability and glass‐transition temperature of PS in PS‐g‐MWNTs were obviously increased. Nanopins were formed on the glass substrates by the self‐assembly of PS‐g‐MWNTs, and the dewetting effect between the glass substrate and PS chains covered MWNTs during the evaporation of the solution. Both the length and diameter of the nanopins increased with the solution concentration. When PS‐g‐MWNTs were compression‐molded, MWNTs were dispersed uniformly in the PS matrix and formed good networks, such as circlelike and starlike structures, because of the entanglements of hairy PS chains on MWNTs. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3869–3881, 2006     

Atomic Force Microscopic Studies of Light-Emitting Porous Silicon

The surface morphologies of porous silicon (PS), fabricated under various anodic etching conditions and on different typvs of silicon substrates, were studied using atomic force microscopy (AFM). The typical sizes of silicon crystallites of PS were found to be 4-10 nanome-ters,an expected for quantum confinement effect. The results suggested the doping concentration of silicon substrate plays an important role compared with the doping type in determining the surface microstructures. On p^- PS samples, a step-like dependence of the PS crystallite size of HF etching concentration was observed for the first time,in nice agreement with the,tep-like photo-luminescence (PL) phenomenon. which strongly supported the quantum confinement model.     

CVD growth of N-doped carbon nanotubes on silicon substrates and its mechanism

In the present study, we report the chemical vapor deposition (CVD) of nitrogen-doped (N-doped) aligned carbon nanotubes on a silicon (Si) substrate using ferrocene (Fe(C5H5)2) as catalyst and acetonitrile (CH3CN) as the carbon source. The effect of experimental conditions such as temperature, gaseous environment, and substrates on the structure and morphology of N-doped carbon nanotubes arrays is reported. From XPS and EELS data, it was found that the nitrogen content of the nanotubes could be determined over a wide range, from 1.9% to 12%, by adding the addition of hydrogen (H2) to the reaction system. It was also shown by SEM that N-doped carbon nanotube arrays could be produced on Si and SiO2 substrates at suitable temperatures, although at different growth rates. Using these concentrations, it was possible to produce three-dimensional (3D) carbon nanotubes architectures on predetermined Si/SiO2 patterns. The mechanism underlying the effect of nitrogen containing carbon sources on nanotube formation was explored using X-ray photoelectron spectroscopy (XPS).     

Carbon nanotube network formation from evaporating sessile drops

Fabrication of single-walled carbon nanotube (SWNT) networks using evaporation of SDS-SWNT sessile drops on a hydrophobized silicon substrate is reported. It is suggested that the organization of nanotubes during evaporation is controlled by aggregates (in the SDS-SWNT dispersion) and hydrophobicity of the substrate. On hydrophobic substrates, the evaporation of SDS-SWNT sessile drops proceeds through constant contact area. On hydrophilic substrates, nanotube aggregates in SDS-SWNT dispersion stop the contact line from moving, resulting in the formation of "coffee-stains". The (partial) removal of aggregates by centrifugation is essential for a freely moving contact line leading to the organization of nanotubes into a network of homogeneously distributed nanotubes on the most hydrophobic substrate. The evaporation of sessile drops was characterized by microscopic, spectroscopic, and topographical techniques.     

Growth control of single and multi-walled carbon nanotubes by thin film catalyst

Single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) were synthesized on silicon substrate by the control of catalyst size, hydrocarbon species, and carbon flux through chemical vapor deposition (CVD). Catalysts for SWNTs and MWNTs could be obtained by an agglomeration of sputtered Co–Mo and pure Co thin films, respectively. The addition of Mo in the Co catalyst provides an effective nucleation site for SWNT and the low carbon flux by using methane gas in CVD reaction makes it possible to grow a single-walled structure.     

Films and composites of gold nanoparticles stabilized by abietic acid

We studied the usefulness of dispersions of gold nanoparticles (Au-NPs) stabilized by abietic acid for manufacturing films on glass and silicon substrates and composites with nanosized and powder materials exemplified by SiO₂ and TiO₂ NPs, expanded graphite (EG), carbon nanotubes (CNTs), a porous carbon material (PCM), PCM containing 6% nitrogen, and CeO₂ powder.     

Singlewall carbon nanotubes covered with polystyrene nanoparticles by in‐situ miniemulsion polymerization

This work is to make carbon nanotubes dispersible in both water and organic solvents without oxidation and cutting nanotube threads. Polystyrene‐singlewall carbon nanotube (PS‐SWNT) composites were prepared with three different methods: miniemulsion polymerization, conventional emulsion polymerization, and mixing SWNT with PS latex. The two factors, crosslinking and surface coverage of PS are important factors for the mechanical and electrical properties, including dispersion states of SWNT in various solvents. The PS‐SWNT composite prepared via a conventional emulsion polymerization showed SWNT bundles entirely covered with PS, whereas the PS‐SWNT composite prepared via a miniemulsion polymerization showed SWNT partially covered with crosslinked PS nanoparticles. The method of mixing SWNTs with PS latex did not show the well dispersed state of carbon nanotubes because PS was not crosslinked and was dissolved in a solvent, and nanotubes separated from PS precipitated. So the PS nanoparticle‐SWNT composite had lower electrical resistance, and higher mechanical strength than the other composites made by the latter two methods. As the amount of SWNT increases, the bare surface area of SWNT increases and the electrical conductivity increases in the composite made by the miniemulsion polymerization. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 573–584, 2006     

Growth of Single-walled Carbon Nanotubes on Substrates Using Carbon Monoxide as Carbon Source

The growth of single-walled carbon nanotubes(SWCNTs) on substrates has attracted great interests because of the potential applications in various fields. Carbon monoxide(CO) was used as the carbon source for the growth of SWCNTs on silicon substrates. Random or oriented SWCNTs can be produced by varying the CO flow rate. When the flow rate of CO was as low as 20 sccm(sccm:standard cubic centimeter per minute), dense SWCNT networks with clean surface were produced. When the flow rate was above 50 sccm, vertically aligned SWCNT(VA-SWCNT) arrays were grown. Well-aligned VA-SWCNT arrays were obtained in the temperature range of 650-800℃ and the content of large-diameter(above 1.7 nm) tubes in the array increased with the temperature. The height of the array was affected by the growth temperature, the CO flow rate, and the growth time. These findings indicate CO can be used as an efficient carbon source for the growth of SWCNTs on substrates under low flow rates.     

碳纳米管@碳化硅同轴异质结纳米管非金属光催化剂产氢性能（英文）

近年来,光催化裂解水产氢(H2)引起了广泛的关注.储量丰富,环境友好的非金属无机半导体β-SiC(立方相碳化硅)具有适当的带隙(Eg=2.4 eV,ECB=?0.9 V),是一种潜在的光催化剂.受限于SiC光催化剂内部光生电子-空穴对的快速复合,SiC光催化剂的效率较低.已有的关于SiC光催化剂改性的报道主要包括构建纳米SiC,构建SiC异质结,构建碳/SiC材料杂化材料.进一步的研究表明,SiC与碳材料之间通过紧密的界面接触形成了肖特基结,能将SiC表面的光生电子快速转移,抑制光生电子-空穴对的快速复合,从而提高光催化分解水产氢的活性.另一方面,碳纳米管(CNTs)具有良好的电子导电性,一维有序的管腔所形成的电子快速传导路径.因此,将半导体光催化剂与CNTs复合,是一种制备先进的光催化剂的有效策略.本文利用Si蒸气与CNTs之间的气-固反应,在CNTs表面原位生长SiC纳米包覆层,成功地制备了一维同轴核-壳CNTs@SiC纳米管.高分辨率透射电子显微镜图像表明,SiC与CNTs之间是通过Si-C共价键原子接触,并得到X射线光电子能谱的证实.将一部分CNTs@SiC纳米管在空气中750 oC煅烧2 h以除去CNTs,得到纯SiC纳米颗粒作为对比组.紫外-可见吸收光谱表明,CNTs能够促进SiC对光的吸收.荧光发射光谱(PL),瞬态荧光寿命测试,瞬态光电流测试以及交流阻抗(EIS)测试表明,CNTs能够促进SiC表面光生电子的传输与分离,有利于提升光催化效率.以0.1 mol/L Na2S溶液作为牺牲剂,在模拟太阳光(A.M 1.5)照射下,CNTs@SiC纳米管(不额外负载Pt等贵金属作为助剂)的产氢速率为118.5μmol g^-1 h^-1,是纯SiC纳米颗粒(21.1μmol g^-1 h^-1)的5.62倍.经过20 h的光照测试,CNTs@SiC纳米管的光催化性能无明显衰减;X射线衍射测试与扫描电子显微镜图像表明,CNTs@SiC纳米管的结构与形貌反应前后几乎无变化.莫特-肖特基测试表明,CNTs的费米能级比SiC低,因此SiC表面的光生电子能够快速地转移到CNTs,并且CNTs的良好导电性与一维有序的管腔所形成的长的电子传导路径能够进一步地增加电子寿命,促进光生电子参与光催化反应.另外,通过原子连接的同轴核-壳CNTs@SiC纳米管提供了大量且有效的电子传输路径.因此,与纯SiC纳米晶等同类材料相比,无机非金属CNTs@SiC纳米管具有更强的光催化氢活性.     

Influences of organometallic polymer‐derived catalyst dispersion on SWNT growth

Catalyst formation kinetics of a ferrocene‐containing homopolymer, polyferrocenylethylmethylsilane (PFEMS), is investigated as it relates to the catalysis of single walled carbon nanotubes (SWNTs) through a chemical vapor deposition (CVD) process. The formation and efficiency of the PFEMS‐based iron catalyst is compared with that of the corresponding polystyrene (PS)‐b‐PFEMS diblock copolymer. The PFEMS homopolymer contains 23 wt % iron, while PS‐b‐PFEMS, with a 25 vol % PFEMS content, is only 6% iron. Despite its lower iron content, spin‐cast PS‐b‐PFEMS films on SiO₂/Si substrates produce more active iron sites than spin‐cast PFEMS films during CVD growth of SWNTs. This is related to the self‐assembly of the block copolymer, where PFEMS domains are well dispersed in the PS matrix, which degrades at a CVD temperature of 920 °C to leave catalytically active elemental iron behind. On the contrary, the pure PFEMS films contain a high percentage of iron and silicon, which tend to transform into ceramic‐coated iron at this high temperature, thus rendering the iron inactive towards SWNT growth. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 758–765, 2007     

Oxygen-aided synthesis of polycrystalline graphene on silicon dioxide substrates

We report the metal-catalyst-free synthesis of high-quality polycrystalline graphene on dielectric substrates [silicon dioxide (SiO(2)) or quartz] using an oxygen-aided chemical vapor deposition (CVD) process. The growth was carried out using a CVD system at atmospheric pressure. After high-temperature activation of the growth substrates in air, high-quality polycrystalline graphene is subsequently grown on SiO(2) by utilizing the oxygen-based nucleation sites. The growth mechanism is analogous to that of growth for single-walled carbon nanotubes. Graphene-modified SiO(2) substrates can be directly used in transparent conducting films and field-effect devices. The carrier mobilities are about 531 cm(2) V(-1) s(-1) in air and 472 cm(2) V(-1) s(-1) in N(2), which are close to that of metal-catalyzed polycrystalline graphene. The method avoids the need for either a metal catalyst or a complicated and skilled postgrowth transfer process and is compatible with current silicon processing techniques.     

Facile and controllable assembly of multiwalled carbon nanotubes on polystyrene microspheres

Herein a facile and controllable heterocoagulation between polystyrene （PS） microspheres and multiwalled carbon nanotubes （MWCNTs） is introduced based on colloid thermodynamics. The MWCNTs play the role of steric stabilizer for stabilizing the metastable PS microspheres and thus immobilize spontaneously on the surface of PS microspheres. The synthesized MWCNTs-coated PS composite particles have been extensively characterized by scanning electron microscopy, transmission electron microscopy, thermogravimetry and Raman spectroscopy. The results indicate that the structure and morphology of the resultant MWCNTs-coated PS composite particles are significantly affected by the weight ratio of PS and MWNCTs and the amount of poly（vinylpyrrolidone） that is injected into PS dispersion before they are mixed with MWCNTs. Therefore, these composite particles have the potential to produce MWCNTs-based composite materials with controllable mass loading and dispersity of MWCNTs.     

Templated nanostructured PS‐b‐PEO nanotubes

With anodic aluminum oxide (AAO) membranes as wetting templates, nanotubes of the cylinder‐forming polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) copolymer were generated. The PS‐b‐PEO solution was introduced into the cylindrical nanopores of an AAO membrane by capillary force and polymeric nanotubes formed after solvent evaporation. Because of the water solubility of the cylindrical PEO microdomains and the orientation of the cylindrical PEO microdomains with respect to the nanotube walls, the nanotubes were permeable to aqueous media. PS‐b‐PEO nanotubes were also prepared on the interior walls of amorphous carbon nanotubes (a‐CNTs). Because of the unique water permeability of the PEO microdomains, an avenue for functionalizing the interior of the a‐CNTs is enabled. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2912–2917, 2007     

Engineering Hierarchical Nanostructures by Elastocapillary Self‐Assembly

Surfaces coated with nanoscale filaments such as silicon nanowires and carbon nanotubes are potentially compelling for high‐performance battery and capacitor electrodes, photovoltaics, electrical interconnects, substrates for engineered cell growth, dry adhesives, and other smart materials. However, many of these applications require a wet environment or involve wet processing during their synthesis. The capillary forces introduced by these wet environments can lead to undesirable aggregation of nanoscale filaments, but control of capillary forces can enable manipulation of the filaments into discrete aggregates and novel hierarchical structures. Recent studies suggest that the elastocapillary self‐assembly of nanofilaments can be a versatile and scalable means to build complex and robust surface architectures. To enable a wider understanding and use of elastocapillary self‐assembly as a fabrication technology, we give an overview of the underlying fundamentals and classify typical implementations and surface designs for nanowires, nanotubes, and nanopillars made from a wide variety of materials. Finally, we discuss exemplary applications and future opportunities to realize new engineered surfaces by the elastocapillary self‐assembly of nanofilaments.     

Rheological, thermal, and mechanical characterizations of polystyrene/single-walled carbon nanotube composites

For preparation of polystyrene (PS) composites, a polymeric dispersant, pyrene-capped polystyrene (PyPS), was applied for noncovalent functionalization of single-walled carbon nanotubes (SWNTs) to improve both dispersion quality and PS–SWNT interfacial interactions. To demonstrate the critical role of PyPS, the composites with the absence of PyPS (PS/SWNT) were also prepared for comparison. Rheological studies suggest that addition of SWNTs, particularly of PyPS-functionalized SWNTs, suppresses significantly large-scale relaxation of PS chains but has little effect on their short-range dynamics. Relative to PS, moderately improved thermal and mechanical properties took place on the composites with either pristine or PyPS-functionalized SWNTs. The PS/PyPS/SWNT composite usually presents better performance than the PS/SWNT one at a fixed SWNT content.     

多孔硅表面钝化对其发光性能的影响

报道多孔硅（PS）的表面钝化对其光致发光（PL）和电致发光（EL）的影响。PL和EL谱表明,经钝化处理的PS的PL和EL强度明显增强,且发光峰位较大蓝移;存放实验表明,经钝化处理的PS的PL和EL发光强度和发光峰位具有较好的稳定性;I～V曲线显示,经钝化处理的PS发光器件具有较低的启动电压。这些结果表明：用钝化处理的方法是提高PS的PL和EL强度和稳定性及改善其器件性能的有效途径。     

Bulk polymerized polystyrene in the presence of multiwalled carbon nanotubes

The in situ bulk polymerization method was applied to synthesize composites of multiwalled carbon nanotubes (MWNTs) and polystyrene (PS) under ultrasonication to open π-bonds in the MWNTs. Morphology of the composite products was studied by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Thermal properties and molecular weight of the PS synthesized in the presence of the MWNTs were examined by thermogravimetric analysis (TGA) and gel permeation chromatography (GPC), respectively. The MWNTs were observed to play an important role as initiator consumers during the polymerization reaction. Electrical conductivity of a film-type sample of the PS/MWNT nanocomposite was found to increase with increased amount of MWNTs added, following the percolation theory.